In part thanks to margins of error within the poll, partisan loyalty is a better guide to the overall results than is the split among independents. It’s why Trump’s trouble consolidating Republican voters has been a problem for him since the conventions. Partisan loyalty matters.
Back to the issue at hand: Why is national polling all over the map? A number of reasons, including that pollsters don’t agree on who will go vote. Trende’s point isn’t that the results are a question mark, it’s that there’s a valid question about why pollster results aren’t lining up. Does this mean that Trump is right, and we could see a “Brexit” result (by which Trump means an upset, which Brexit wasn’t really)? If the race is close on Election Day, sure. If Clinton continues to lead by five or more points? Probably not.